This invention relates to apparatus for measuring displacement between a scale having a periodic structure and a read head.
A known type of such apparatus comprises a plurality of gratings, one of which is defined by the periodic structure of the scale. An illumination means interacts with one or more of the gratings to produce an interaction product including a spatially periodic fringe field. Another of the gratings forms an analyser grating situated in a position co-planar with the fringe field and having a periodicity which is substantially the same as the periodicity of the fringe field, whereby the fringe field and the analyser grating co-operate to produce light modulations when, in operation, the scale and the read head are displaced one relative to the other. Means are provided for detecting the light modulations in order to measure the displacement. Examples of such apparatus are shown in U.S. Pat. No. 3,812,352 (MacGovern) and in Diffraction Gratings, M. C. Hutley, Academic Press, 1982, pages 293-304.
For example, the known apparatus may include a source of light arranged for illumination of the periodic structure of the scale and interacting therewith to produce the interaction product including the fringe field, the analyser grating being provided in the read head. Alternatively, the apparatus may comprise a source of light arranged for illumination of the periodic structure of the scale and interacting therewith to produce a first interaction product, an index grating positioned in the light path of the first interaction product and interacting therewith to produce a second interaction product including said fringe field, both the index grating and said analyser grating being provided in the read head.
It is expensive to provide gratings in the read head, and so it would be advantageous to avoid the use of a grating in the read head or to reduce the number of such gratings.